1. Field of the Invention
The present invention relates to an anti-reflective substrate and, more particularly, to an anti-reflective substrate with highly rugged surface and the method of manufacturing the same.
2. Description of Related Art
Currently, the energy source of an optoelectronic device is from electrons transferred by photo-electric conversion. The less the light reflective ratio of the device is (more light is absorbed by the surface of the device), the higher the intensity can be obtained. Therefore, the investigation of methods for producing anti-reflective layers of optoelectronic devices with high anti-reflective efficiency is very important.
Among the conventional methods to receive more light, one is to enhance the transmittance of incidental light by forming an anti-reflective membrane on the surface of optoelectronic devices, and another one is to etch the substrate to increase the surface area by etched porous.
There are several methods for etching the surface of the substrate. “Poly silicon dry etching” is to form a poly silicon layer on the silicon wafer first, and etch by high density plasma etching system. However, the system is very rarely used because of high cost, and additional preparation steps of the poly silicon material are required. Therefore, the shortcomings of high cost and complex process limit the use of the method. “Poly silicon wet etching” has the same shortcomings as dry etching described above. Referring to FIG. 1, the results of the substrate surface treated with “poly silicon dry etching” and “poly silicon wet etching” separately are shown. FIG. 1A shows the absorption of the poly silicon substrate surface after dry etching, and FIG. 1B shows that from poly silicon substrate surface after wet etching. Both of the two results only show a better absorption at wavelength of 700 nm. This indicates that the two methods are not able to absorb the light within a broad wavelength range, and this might also be a shortcoming of these conventional methods.
Another method for treating the substrate is “wet etching” the surface of the silicon wafer directly. Nevertheless, the etched surface is not adequately smooth because the etching state is difficult to control.
An electrochemical method is also used to form anti-reflective layers directly on substrates. In the electrochemical method, a bias is applied under the wafer, and a direction is provided whereby the wafer is etched. However, the method incurs the same defects as the wet etching method does, i.e., it is very difficult to control the etching state, and only a nearly smooth surface is eventually obtained.
The conventional methods described above suffer from the difficulty to control the etching state, and thus increase the complexity of the process. Further, the devices produced from the conventional method above can be operated within a narrow range of wavelength, that means the reflective efficiency of the devices is high, and the energy transferring efficiency is low because most light is reflected from the device. Hence, a reflective layer having a high absorption within a broad range of wavelength is beneficial to keep light inside the device. Thus, there is an urgent need for a reflective layer with high absorption, and the method for making the reflective layer to be developed.